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Based on the interface polarization model, the two-dimensional electron gas (2DEG) at $LaInO_{3}$(LIO)/$BaSnO_{3}$(BSO) interfaces is understood to originate from a polarization discontinuity at the interface and the conduction band offset between LIO and BSO. In this scenario, the direction of polarization at the interface is determined by whether the first atomic LIO layer at the interface is LaO$^{+}$ or InO$_{2}^{-}$. We investigate the role of the terminating layer at the LIO/BSO interface in creating the 2DEG. Based on conductance measurements of our in-situ grown LIO/BSO heterostructures, we report in this work that the 2DEG only forms when the BSO surface is terminated with a SnO$_{2}$ layer. We controlled the terminating layer by additional SnO$_{2}$ deposition on the BSO surface. We show that the as-grown BSO surface has a mixed terminating layer of BaO and SnO$_{2}$ while the BSO surfaces prepared with additional SnO$_{2}$ deposition are terminated mainly with the SnO$_{2}$ layer. The terminating layer was confirmed by coaxial impact collision ion scattering spectroscopy (CAICISS). Our finding is consistent with the interface polarization model for 2DEG formation at LIO/BSO interfaces, in which the direction of the interfacial polarization in LIO is determined by the terminating layer of the BSO surface., 22 pages

Spontaneous noble metal dopant segregation in an oxide lattice can lead to the formation of metallic clusters and extended acicular inclusions. In a thin-film process, the shape and orientation of such noble metal inclusions are governed by the crystal growth direction, giving rise to a composite material with lattice-matched metal nanopillars embedded vertically in an insulating or semiconducting oxide matrix. An interesting application of such composites is in photoelectrochemical cell electrodes, where the metallic nanopillars take on three distinct roles: forming a Schottky junction with the host matrix, providing a low-loss current path from bulk to surface, and creating an efficient electrocatalytic active site on the electrode surface. In particular, we discuss the application of vertically aligned metal–oxide nanopillar composites in photoelectrochemical water-splitting cells used for direct solar-powered hydrogen generation.

The use of a sacrificial water-soluble BaO layer was investigated for the purpose of fabricating free-standing single-crystalline BaTiO3 membranes for vibrational energy harvesting applications. An epitaxial BaTiO3/SrTiO3/BaO heterostructure was deposited on a SrTiO3(001) substrate by pulsed laser deposition. The sacrificial BaO layer was dissolved by immersing the heterostructure in water. A single-crystalline BaTiO3 membrane was thereby released from the SrTiO3(001) substrate and transferred onto a glass or polyethylene terephthalate (PET) substrate coated with a polydimethylsiloxane (PDMS) adhesion layer. A 200 nm-thick BaTiO3 membrane was sandwiched between two flexible PDMS/ITO/PET electrode layers, forming a vibrational energy harvester test device that could operate at an average electrical power output of over 1 μW. This membrane fabrication process is applicable to various flexible structures and materials in vibrational energy harvesting or dynamic strain-sensing applications.

Photoelectrochemical solar water splitting is a potentially useful way to store solar energy in a renewable fuel in the form of hydrogen. For practical use of an efficient and reliable system, a qu...

A quantum spin Hall insulator hosts topological states at the one-dimensional edge, along which backscattering by nonmagnetic impurities is strictly prohibited and dissipationless current flows. Its 3D analogue, a weak topological insulator (WTI), possesses similar quasi-1D topological states confined at side surfaces of crystals. The enhanced confinement could provide a route for dissipationless current and better advantages for applications relative to the widely studied strong topological insulators. However, the topological side surface is usually not cleavable and is thus hard to observe by angle-resolved photoemission spectroscopy (ARPES), which has hindered the revealing of the electronic properties of WTIs. Here, we visualize the topological surface states of the WTI candidate ZrTe5 for the first time by spin and angle-resolved photoemission spectroscopy: a quasi-1D band with spin-momentum locking was revealed on the side surface. We further demonstrate that the bulk band gap in ZrTe5 is controlled by strain to the crystal, realizing a more stabilized WTI state or an ideal Dirac semimetal state depending on the direction of the external strain. The highly directional spin-current and the tunable band gap we found in ZrTe5 will provide an excellent platform for applications., 8 pages, 4 figures. Accepted by Nature Communications

Dirac and Weyl semimetals with linearly crossing bands are the focus of much recent interest in condensed matter physics. Although they host fascinating phenomena, their physics can be understood in terms of weakly interacting electrons. In contrast, more than 40 years ago, Abrikosov pointed out that quadratic band touchings are generically strongly interacting. We have performed terahertz spectroscopy on the films of the conducting pyrochlore Pr2Ir2O7, which has been shown to host a quadratic band touching. A dielectric constant as large as $$\tilde{ \varepsilon} {\mathrm{/}}\epsilon _0 \sim 180$$ ε ̃ ∕ ϵ 0 ~ 180 is observed at low temperatures. In such systems, the dielectric constant is a measure of the relative scale of interactions, which are therefore in our material almost two orders of magnitude larger than the kinetic energy. Despite this, the scattering rate exhibits a T2 dependence, which shows that for finite doping a Fermi liquid state survives—however, with a scattering rate close to the maximal value allowed.

The use of thermally stable Sr2RuO4 electrodes in high-temperature synthesis of oxide heterostructures was investigated. Atomically smooth Sr2RuO4 thin films were grown on SrTiO3(001) substrates by pulsed laser deposition and used as a bottom electrode for ferroelectric BaTiO3 capacitors grown at temperatures of up to 1000 °C. The thermal stability of Sr2RuO4 electrodes was verified by structural and electrical measurements of the ferroelectric BaTiO3 films. The best growth temperature for the BaTiO3 films was found to be 900 °C, exhibiting the largest spontaneous polarization, dielectric constant, and pyroelectric response. We conclude that Sr2RuO4 films are suitable for use as thermally stable electrodes in heterostructures synthesized at temperatures up to at least 1000 °C and oxygen pressures from 10–6 to 10–1 Torr. This range of growth film conditions is much wider than that for other common oxide electrode materials such as SrRuO3, widening the available process window for optimizing the performance...

We have grown IrO2 thin films on TiO2(110) substrates to determine the pulsed laser deposition growth window for iridates. Relaxed IrO2 films were obtained at a growth temperature of 500 °C and background oxygen pressure of 100 m Torr; otherwise, either pure Ir metal films or evaporative Ir loss were observed. Although x-ray Φ-scan measurement indicated that the films were epitaxial, a distinct grain structure was seen by atomic force microscopy and transmission electron microscopy. The grain boundaries were found to limit the conductivity of films at low temperature. It appeared that strain relaxation leads to stacking faults at grain boundaries.

The wettability of solid surfaces is of fundamental scientific interest and related to many diverse chemical and physical phenomena at the heart of practical technologies. In particular, the hydrophilicity of the photocatalytically active metal-oxide TiO2 has attracted considerable attention for many applications. However, the intrinsic hydrophilicity of Ti-oxide surfaces is not fully understood. In this work, we investigate the intrinsic hydrophilicity of Ti-oxide surfaces on the atomically stable (√13 × √13)-R33.7° SrTiO3 (001) surface. The surface has a TiOx double layer on a TiO2-terminated SrTiO3 (001) surface, which is available as a surface marker to assess the atomic-scale structural stability of the surface. Both experimental and theoretical results show that Ti-oxide surfaces are intrinsically superhydrophilic with a water contact angle of ∼0°. The results show that airborne surface contamination is the most significant factor affecting the wettability of titania surfaces, strongly supporting th...

Significance Weyl semimetals carry the promise of quantum electronic applications. Theoretical calculations have suggested that Luttinger semimetals recently found in iridium oxides may be a suitable group of materials where such topological phases, including Weyl semimetal state, may be found. For example, praseodymium-iridium oxide, P r 2 I r 2 O 7 , is one such Luttinger semimetal that can be tuned by perturbations such as strain into a Weyl semimetal state. Despite theoretical predictions of Weyl semimetal states in P r 2 I r 2 O 7 crystals, experimental proof remains elusive due to the difficulty of applying sufficient mechanical strain on single crystals. Our study provides strong experimental evidence that a Weyl semimetal state may indeed appear in strained pyrochlore iridium oxide films, opening a way to explore topological phases.

Oxide single-crystal substrates with atomically smooth and chemically uniform surfaces are indispensable for constructing high-quality epitaxial heterostructures and sharp heterointerfaces. In this paper, the authors develop a simple and efficient recipe to optimize the surface structure in LaAlO${}_{3}$ (001) single crystal, a widely used substrate for growing perovskite oxide heterostructures. The authors combine thermal annealing and subsequent deionized water leaching processes to treat the LaAlO${}_{3}$ (001) surface. Thanks to the distinct solubility between AlO${}_{2}$ and LaO surface layers, the treated substrate exhibits an atomically flat and uniformly AlO${}_{2}$-terminated surface. This method circumvents the high-temperature instability of LaAlO3 (001) surface due to the intrinsic surface polarity.

Closed-loop optimization of epitaxial titanium nitride (TiN) thin-film growth was accomplished using metal-organic molecular beam epitaxy (MO-MBE) combined with a Bayesian machine-learning technique and reduced the required number of thin-film growth experiments. Epitaxial TiN thin films grown under the process conditions optimized by the Bayesian approach exhibited abrupt metal–superconductor transitions above 5 K, demonstrating a new approach to the efficient development of less-studied materials, such as transition metal nitrides. The combination of the thin-film growth technique and Bayesian approach is expected to pave the way toward accelerating the development of the automated operation of thin-film growth apparatuses.

Production of chemical fuels by direct solar energy conversion in a photoelectrochemical cell is of great practical interest for developing a sustainable energy system. Various nanoscale designs such as nanowires, nanotubes, heterostructures and nanocomposites have been explored to increase the energy conversion efficiency of photoelectrochemical water splitting. Here we demonstrate a self-organized nanocomposite material concept for enhancing the efficiency of photocarrier separation and electrochemical energy conversion. Mechanically robust photoelectrodes are formed by embedding self-assembled metal nanopillars in a semiconductor thin film, forming tubular Schottky junctions around each pillar. The photocarrier transport efficiency is strongly enhanced in the Schottky space charge regions while the pillars provide an efficient charge extraction path. Ir-doped SrTiO3 with embedded iridium metal nanopillars shows good operational stability in a water oxidation reaction and achieves over 80% utilization of photogenerated carriers under visible light in the 400- to 600-nm wavelength range., Nanoscale designs are known to increase the energy conversion efficiency of photoelectrochemical water splitting. Here, the authors report a self-organized nanocomposite formed by embedding self-assembled metal nanopillars in a semiconductor thin film, for enhanced photocarrier separation efficiency.

Optical pump-THz probe (OPTP) measurements allow us to investigate the dynamic response of photoconductivity in materials by observing the transient change of THz transmittance after optical excitation. In this study, the THz response of d -electrons in various photoexcited states in MnO was investigated by pump-energy-tunable OPTP measurements to elucidate the photoexcited d -electron dynamics in a transition metal oxide. At room temperature, photoexcited d -electrons induced by a d -d transition to the lowest excited d -state showed the longest relaxation time. The relaxation time decreased drastically below the antiferromagnetic transition temperature of 120 K. This decrease was accompanied by the emergence of magnetic-excitation-assisted photoluminescence from a self-trapped exciton (STE) state. This suggests that photoexcited d -electrons relax to the STE state below the transition temperature. These findings shed new light on the photoexcited d -electron dynamics, contrasting with the behavior of photoexcited carriers generated in the upper excited d -states that have been shown in a previous study to relax into trap states through carrier scattering. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We discuss methods of built-in carrier density control in SrTiO3/LaTiO3/SrTiO3 heterostructures that exhibit quasi-two-dimensional carrier confinement in an interfacial quantum well. Unlike the electronically similar LaAlO3/SrTiO3 heterostructures, where the polar discontinuity at the interface defines the accumulated carrier density, the LaTiO3 heterostructures offer two means of carrier density control—changing the La doping level and utilizing the effect of surface depletion through the change in the SrTiO3 capping layer thickness. Dynamic carrier tuning over a limited range is possible by the application of a back-gate bias, which primarily affects the depth distribution of carriers. We find that small changes in the pre-annealing conditions of a SrTiO3 substrate can have a dramatic effect on the low-temperature sheet resistance of the heterostructures.

(111)-oriented pyrochlore Pr 2Ir 2O 7 epitaxial thin films were successfully fabricated on yttria-stabilized zirconia (111) substrates by means of solid phase epitaxy, namely, the combination of pulsed laser deposition (PLD) of an amorphous precursor film at room temperature, followed by epitaxial crystallization in an ex situ post-annealing process in air. The Pr 2Ir 2O 7 phase cannot be obtained in a conventional in situ PLD process because of severe Ir loss. The Pr 2Ir 2O 7 thin films showed metallic electronic conductivity and the spontaneous Hall effect. These transport properties are similar to the ones reported for bulk single crystal samples, indicating that the thin films had high crystalline quality.

International audience; We studied the synthesis of nanocomposite materials consisting of noble metal clusters embedded in an oxide semiconductor matrix. The embedded nanostructures form in a simple self-organized single-step growth process. The primary interest is in developing materials for photo-electrochemical energy conversion where spatially inhomogeneous band structures can enhance photogenerated charge separation and carrier extraction from a semiconductor. We show that spontaneous segregation of metallic Ir occurs during the initial growth of an Ir:SrTiO3 thin film. Cross-sectional transmission electron microscopy suggests that the nanoscale Ir clusters are epitaxial with the host lattice, and their presence is not detectable by surface morphology measurements.

Silver oxide clathrate Ag7O8M (M = NO3, HSO4) compounds were synthesized photoelectrochemically on rutile-type Nb-doped TiO2 single-crystal substrates. Epitaxial crystal growth was achieved for some clathrate compositions and substrate surface orientations, where commensurate growth is possible due to lattice matching between the pseudo lattice of the clathrate Ag6O8 cages and the TiO2 surface, similar to the well-known case of epitaxial C60 growth on single-crystal substrates. Particularly for the growth of Ag7O8NO3 on Nb-doped TiO2(110), fully (111)-oriented epitaxial crystallites without any other orientations were obtained. The selectivity for Ag7O8NO3 growth and the suppression of the formation of by-products, such as O2, were found to depend on the electrode potential. The highest selectivity was obtained at +0.2 V vs. Ag in a 0.01 M AgNO3 solution. An investigation of Ag7O8(MM′) (M = NO3, M′ = HSO4) depositions from solutions with different AgNO3 and Ag2SO4 mixing ratios showed that the growth of Ag7O8HSO4 is much faster than that of Ag7O8NO3. The process of incorporating monovalent M anions into the clathrate Ag6O8 cages was identified as the rate-limiting step for the growth of silver oxide clathrate compounds.

The use of thermally stable Sr

International audience; The structure of Ir-doped LaAlO3/SrTiO3(001) interfaces was investigated on the atomic scale using probe-corrected transmission electron microscopy in high-angle annular dark-field scanning mode (HAADF-STEM) and electron energy loss spectroscopy (EELS), combined with first-principles calculations. We report the evolution of the strain state experimentally measured in a 5 unit-cell thick LaAlO3 film as a function of the Ir concentration in the topmost SrTiO3 layer. It is shown that the LaAlO3 layers remain fully elastically strained up to 3% of Ir doping, whereas a higher doping level seems to promote strain relaxation through enhanced cationic interdiffusion. The observed differences between the energy loss near edge structure (ELNES) of Ti-L2,3 and O-K edges at non-doped and Ir-doped interfaces are consistent with the location of the Ir dopants at the interface, up to 3% of Ir doping. These findings, supported by the results of density functional theory (DFT) calculations, provide strong evidence that the effect of dopant concentrations on the properties of this kind of interface should not be analyzed without obtaining essential information from the fine structural and chemical analysis of the grown structures.

The effect of iridium valence in Ir:SrTiO3 on the electronic structure and the photocatalytic activity in a water splitting reaction was studied. Epitaxial thin film photoelectrodes were grown with controlled Ir valence and used to measure the electrochemical efficiency of Ir:SrTiO3. The positions of the in-gap Ir impurity levels were determined by optical and X-ray photoelectron spectroscopies. Comparison with ab initio calculations was used to assign the observed electronic states to either Ir4+ or Ir3+ dopants in SrTiO3. The measurements show that Ir3+:SrTiO3 forms a single midgap impurity state that is strongly localized, completely quenching the photoelectrochemical response. An anodic photoresponse was seen in Ir4+:SrTiO3 under visible-light illumination up to a wavelength of 600 nm (hν = 2.0 eV). Ir4+:SrTiO3 contains an in-gap state close to the top of the SrTiO3 valence band. The performance of Ir4+:SrTiO3 in electrochemical reactions is compared with cathodic Rh3+:SrTiO3, clearly illustrating the...

Temperature-driven dewetting of a self-template layer has been exploited to spontaneously grow epitaxial magnetic Fe3O4 nanocrystals by pulsed laser deposition. A 10 unit cell thick Fe3O4 template layer was deposited at 400 °C on a perovskite SrTiO3(001) substrate, followed by heating at 1100 °C, which induced dewetting of the Fe3O4 template layer, forming three-dimensional islands that were uniformly distributed over the substrate surface area. These islands were used as seed crystals for the growth of spatially separated quasi-epitaxial Fe3O4 nanopyramids. Structural analysis by scanning transmission electron microscopy and X-ray diffraction revealed incoherent growth of fully relaxed (001)-oriented Fe3O4 nanocrystals on the SrTiO3 (001) substrate. Higher-order epitaxial matching of the substrate and film lattices appears to be responsible for the well-defined in-plane orientation of the Fe3O4(001) nanopyramids despite the large nominal lattice mismatch of −7.5%. The nanopyramid structure strongly affec...

A pulsed laser deposition system with rapid beam deflection (RBD-PLD) by a galvanometer mirror scanner has been developed for alternating ablation of multiple targets with a single laser instrument. In this system, the alternating deposition of different target materials is carried out by scanning the laser beam between the positionally fixed targets with a galvanometer mirror instead of mechanically switching the target positions on a fixed optical path of the laser beam as is done in conventional pulsed laser deposition (PLD) systems. Thus, the "wait" time required for switching target materials to be deposited, which typically takes several seconds in a conventional system, can be made as short as a few milliseconds. We demonstrate some of the advantages of this PLD system in several technologically important aspects of thin film synthesis: (1) fast fabrication of binary alloy films, (2) preparation of natural composition spread libraries, (3) effect of the target switching time on the deposition of volatile compounds, (4) control of the degree of mixing of two different materials in a film, and (5) efficient growth of compositionally graded thin films.

Ferromagnetism at LaAlO3/SrTiO3 interfaces has so far been studied by various experimental methods to determine the magnetic origin of the domain structure. Here, we successfully observe granular f...

The synthesis of (111)-oriented BaTiO3 thin films on Nb-doped SrTiO3 and bilayers of BaTiO3 and La0.7Sr0.3MnO3 on SrTiO3 are investigated. With increasing thickness the films are found to exhibit a decreasing out-of-plane lattice parameter and increased surface roughness. The BaTiO3 films on doped SrTiO3 are found to be relaxed with indications of increasing defect density with increasing thickness. Through piezoresponse force microscopy, pyroelectric measurements, and tunneling electroresistance measurements, indications of ferroelectric behavior are found in (111)-oriented BaTiO3 down to a thickness of 5 nm.

Epitaxial self-assembled ferro(i)magnetic spinel (CoFe2O4 (CFO)) and ferroelectric bismuth layered perovskite (Bi5Ti3FeO15 (BTFO)) pillar-matrix nanostructures are demonstrated on (001) single-crystalline strontium titanate substrates. The CFO remains embedded in the BTFO matrix as vertical pillars (∼50 nm in diameter) up to a volume fraction of 50%. Piezoresponse force microscopy experiments evidence a weak out-of-plane and a strong in-plane ferroelectricity in the BTFO phase, despite previously reported paraelectricity along the c-axis in a pure BTFO film. Phenomenological Landau-Ginzburg-Devonshire-based thermodynamic computations show that the radial stress induced by the CFO nanopillars can influence these ferroelectric phases, thus signifying the importance of the nanopillars. The CFO pillars demonstrate robust ferromagnetic hysteresis loops with little degradation in the saturation magnetization (ca. 4 μB/f.u.). Thus BTFO-CFO nanocomposites show significant promise as a lead-free magnetoelectric materials system.

In crystalline thin film growth a prerequisite is substrate surfaces with a stable and uniform structure and chemical composition. Various substrate treatments were used to obtain atomically smooth, step-and-terrace (111)-oriented SrTiO3 with uniform cation layers at the surface, i.e., single termination. The surface control enables subsequent layer-by-layer epitaxial growth of perovskite thin films of La0.7Sr0.3MnO3, LaFeO3, and BaTiO3. Reflection high-energy electron diffraction and electron energy loss spectroscopy revealed that a single chemically intermixed (A,A′)BO3 perovskite layer formed at the interface. As the terminating layer of (111) SrTiO3 is polar, a surface reconstruction consisting of TiOx surface layers is expected, and the intermixing at the interface can be understood as A′-cations from the film material compensating an A-cation deficient substrate surface during initial growth. This finding has important consequences for engineered interfaces between perovskite thin films and polar substrate facets.

A high-performance spin filter tunnel junction composed of an epitaxial oxide heterostructure is reported. By independently controlling the magnetic orientations of ferromagnetic tunnel barrier and electrode layers, a tunnel magnetoresistance ratio exceeding 120% is obtained purely by the spin filtering effect. A newly introduced spin filter material, Pr0.8Ca0.2Mn1-yCoyO3, is shown to be useful for building novel multibarrier spintronic tunnel devices due to its composition-controlled magnetic hardness.

We have investigated self-template control of Fe3O4 thin film orientation on SrTiO3(001) substrates. The growth orientation of Fe3O4 films on SrTiO3(001) is dependent on the preparation temperature, with a crossover from the (001) to (111) grain orientation occurring at around 600 °C. In order to grow high-quality (001)-oriented Fe3O4 thin films at high temperature, a self-template technique was used, where an 8-nm-thick nucleation layer was deposited on a SrTiO3(001) substrate at 400 °C, followed by main film growth at 700 °C. This method achieved films that showed pure (001) grain orientation with bulk-like magnetic and transport behavior.

Flux-mediated epitaxy of self-assembled ferroelectric Bi4Ti3O12 (BIT) nanoplates on SrTiO3 (001) is investigated with vanadium oxide (VOx) as the flux medium. It is shown that precoverage of a SrTiO3 (001) substrate with VOx helps to nucleate thin rutile TiO2 (001) crystallites that act as templates for the subsequent growth of (110)-oriented BIT nanoplates. Piezoresponse force microscopy reveals the presence of 90° domain walls even within a single BIT nanoplate, similar to BIT bulk single crystals.

We have fabricated epitaxial Pr 0.8 Ca 0.2 MnO 3 (PCMO) insulator layers sandwiched between Al top electrodes and epitaxial La 0.6 Sr 0.4 MnO 3 bottom electrode layers on (LaAlO 3 ) 0.3 –(Sr 2 AlTaO 6 ) 0.7 (1 0 0) substrates. Various sizes of metal/insulator/metal device structures were formed by photolithography and Ar ion milling. Device size dependence of Al/PCMO interface resistances was well fitted by series-parallel equivalent circuit, indicating several types of different resistance components exist at the Al/PCMO interfaces. These different resistance components suggest that defects might distribute inhomogeneously at the Al/PCMO interfaces which exhibit the resistance switching.

We have studied the effect of deposition conditions and strain on the phase stability, grain structure, grain orientation, crystallinity, and magnetism of InFeCoO 4 spinel thin films grown on single-crystal (0 0 1) SrTiO 3 (STO) and (0 0 1) MgO substrates. The growth window for good-quality films exhibiting uniform orientation was observed to be very narrow with deviations from optimal conditions resulting in highly disordered growth characterized by mixed (0 0 1) and (1 1 1) type orientations, expanded out-of-plane lattice constants, and reduced symmetry for the (0 0 1) oriented grains. Bulk-like magnetic properties could be obtained in films grown on STO under optimal conditions, whereas significantly lower values for saturation magnetization and T C were observed for the disordered samples. The absence of frequency-dependent components in AC-magnetization measurements indicates the absence of a glassy state previously observed in bulk InFeCoO 4 . The significantly smaller grain size of the samples grown on MgO at optimal conditions resulted in depressed T C in addition to a strong superparamagnetic effect below 20 K.

We review the performance of the pyroelectric measurement method in the context of polar states in multiferroic materials in which ferroelectricity coexists with ferromagnetism. The unique point of pyroelectric measurement is the ability to determine the presence of dielectric polarization without having to apply a bias voltage during the measurement. The convenience of the zero-bias technique is illustrated by summarizing experiments on measuring the pyroelectric hysteresis loops of leaky ferroelectric BaTiO3 and PbTiO3 thin film capacitors. This technique was also employed for detecting a polar state in magnetic materials, proving that the electronic polarization in ferrimagnetic Fe3O4 appears at the Verwey transition temperature at 120 K and to discover A-site-driven ferroelectricity in epitaxially strained ferromagnetic La2NiMnO6 films.

We have fabricated epitaxial La 1.2 Sr 1.8 Mn 1.7 Ru 0.3 O 7 thin films with n =2 Ruddlesden–Popper (RP) structure on TiO 2 -terminated and SrO-buffered SrTiO 3 (0 0 1) single-crystal substrates. It is shown that even small deviation from the narrow growth window results in the nucleation of (AO) 2 double layers in random directions and the loss of the desired structure. The atomic layer ordering at the interface between the thin films and SrTiO 3 substrates was studied by high-pressure, in situ reflection high-energy electron diffraction and transmission electron microscopy. It is demonstrated that growth on TiO 2 -terminated SrTiO 3 commences with the nucleation of two perovskite sub-units before the first (AO) 2 double-layer forms. To obtain a sharp RP/SrTiO 3 interface structure, it is necessary to buffer the TiO 2 -terminated SrTiO 3 substrate with a sub-unit cell SrO layer before the deposition of the RP film.

We have investigated the polarity of zinc oxide (ZnO) and Al-doped ZnO films grown on (11¯20) and (0001) sapphire substrates, using coaxial impact collision ion scattering spectroscopy. The films grown by pulsed laser deposition with a nominally undoped ZnO ceramic target had a (000¯1) surface, whereas the films prepared with a 1 mol% Al-doped ZnO ceramic target had a (0001) surface. The usage of Al-doped and undoped targets caused no difference in the in-plane lattice orientation. Electron microscope observations revealed that polarity change due to doping occurred without the formation of any interfacial phase between ZnO and sapphire.

Synthesis of continuous composition-spread (CCS) thin films is widely recognized in combinatorial material science as a powerful technique for rapidly investigating the properties of new functional materials. However, there are very few reports of CCS organic thin films due to the fact that the thermal evaporation method with Knudsen cells, which is commonly used to fabricate organic thin films in vacuum, does not offer the necessary level of deposition rate control as, for example, does pulsed laser deposition (PLD). We have successfully fabricated organic CCS thin films of pentacene and 6,13-pentacenequinone by continuous-wave laser molecular beam epitaxy (CWL-MBE), which we developed as a new fabrication method for organic thin films. The composition-spread films were characterized systematically by ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, atomic force microscopy, and two-probe conductivity measurements. The present work brings the advantages of high-throughput parallel synthesis and systematic characterization to the field of organic electronics, allowing for quick exploration and rapid optimization of organic functional materials and devices.

We have used photoelectron emission microscopy (PEEM) and X-ray magnetic circular dichroism (XMCD) to study the effect of thin film thickness on the magnetic domain formation in La 0.6 Sr 0.4 MnO 3 samples that were epitaxially grown on stepped SrTiO 3 (0 0 1) substrates. The magnetic image exhibited a stripe structure elongated along the step direction, irrespective of film thickness, suggesting that uniaxial magnetic anisotropy induced by step-and-terrace structures plays an important role in the magnetic domain formation. Additional domains evolved gradually with increasing film thickness. In these domains, the direction of magnetization differed from the step direction due to biaxial magneto-crystalline anisotropy. The evolution of additional magnetic domains with increasing film thickness implies that a competition exists between the two anisotropies in LSMO films.

We have developed a process for fabricating top-gate SrTiO3 field-effect transistors (FETs) where the critical channel interface is constructed before forming the source and drain electrodes in order to reduce the density of defects in the transistor channel. Metallic source and drain electrodes are formed by Argon ion milling, which is an efficient way of introducing oxygen vacancies into a thin surface layer of SrTiO3. These vacancies function as donors, inducing metallic conductivity in the electrode regions. This technique can be used to obtain clean interfaces, suppress the formation of lattice imperfections, and avoid impurities in the FET channel because the electronically active interface is constructed at an early step of the device fabrication process. Metallic conductivity of the SrTiO3-δ source and drain electrodes was maintained even after annealing the device at up to 600 °C. Contact with the transistor channel remained ohmic at least down to 50 K.

We have performed an in situ photoemission spectroscopy (PES) of well-ordered surfaces of high-quality epitaxial La1−x Srx MnO3(LSMO) thin films to investigate the temperature-dependent changes in the electronic structure of LSMO in detail. For ferromagnetic metal LSMO with x = 0.4 , the spectral intensity of the e g states near the Fermi level ( E F ) significantly increases with decreasing temperature. This result suggests that the stabilization of ferromagnetic ordering causes the enhancement of spectral weight near E F .

The pulsed laser ablation and deposition (PLD) processes of SrTiO3 were studied as a model system for developing a technique for the growth of structurally bulk-equivalent epitaxial thin films of typical complex oxides. Deposition rate vs. laser fluence measurements showed that there is a second critical fluence value above the ablation threshold. Below the critical value, both the ablation and deposition rates are linear functions of the fluence and independent of the ablation spot area. Above the critical value, the deposition rate per unit ablation area depends strongly on the ablation spot area. Homoepitaxial SrTiO3 films possessed the exact bulk lattice constant value only when the films were grown at the critical fluence, whereas larger lattice constants were obtained if the fluence was either higher or lower. Composition analysis revealed that the enlargement of the lattice constant was related to cationic unbalance. This critical laser fluence is essential for the growth of bulk-equivalent epitaxial films, and therefore accurate measurement of laser energy and ablation area is vital for reproducible film growth.